The invention relates to a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions, and to a process for producing them, and to their use as a substrate material for coating.
It is known that glasses from the system Li2Oxe2x80x94Al2O3xe2x80x94SiO2 can be converted into glass-ceramics comprising beta-quartz solid solutions and/or keatite solid solutions as the main crystal phases. These glass-ceramics are produced in a plurality of stages. After melting and hot-shaping, the material is usually cooled at temperatures in the region of the transformation temperature (Tg), in order to eliminate thermal stress. The material is cooled further to room temperature. The specified quality features of the glass body are investigated.
A second controlled heat treatment is used to crystallize the starting glass and convert it into a glass-ceramic article. This ceramicization takes place in a multistage thermal process in which first of all, by nucleation at temperatures of 600xc2x0 C. to 800xc2x0 C., nuclei are produced from TiO2 or ZrO2/TiO2 solid solutions. SnO2 may also be involved in the nucleation. During the subsequent temperature rise, beta-quartz solid solutions grow on these nuclei at the crystallization temperature of 700xc2x0 C. to 900xc2x0 C. As the temperature rises further, in the range from 800xc2x0 C. to 1100xc2x0 C., these beta-quartz solid solutions are further transformed into keatite solid solutions. Depending on the composition, the stability range of the glass-ceramic containing beta-quartz solid solution is extensive. With some compositions, the temperature of transition to the keatite solid-solution phase lies up to 150xc2x0 C. higher than the crystallization temperature of the beta-quartz solid solution glass-ceramic. With other compositions, the beta-quartz solid solutions are converted into keatite solid solutions almost without any transition. The transition to keatite solid solution is associated with crystal growth, therefore with increasing crystallite size. This leads to increasing light scattering. The light transmission is reduced to an increasing extent. As a result, the glass-ceramic article appears increasingly translucent and ultimately opaque. The high light transmission of the glasses and glass-ceramics allows effective assessment of quality. Shaped bodies with defects which are relevant to safety or jeopardize the specified product properties can be sorted out prior to further process steps.
A key property of these glass-ceramics is that it is possible to produce materials which have an extremely low coefficient of thermal expansion in the range from 20xc2x0 C. to 300xc2x0 C. and above of  less than 1.5xe2x80xa210xe2x88x926/K. With glass-ceramics which contain beta-quartz solid solutions as the main crystal phase, even materials with virtually no expansion are obtained in this temperature range. For use as substrate material for reflectors used in astronomy, glass-ceramics are modified in such a way that their zero thermal expansion lies in the temperature range of xe2x88x9250xc2x0 C. to +50xc2x0 C. which is important for this application. A glass-ceramic material of this type is produced under the name ZERODUR at SCHOTT GLAS.
A recent development is for these glass-ceramics also to be used in illumination engineering as a material for reflectors in applications in which, on account of miniaturisation and high luminous powers, too high thermal loads occur. Compared to the widespread reflectors made from borosilicate or aluminosilicate glass, these glass-ceramics satisfy extremely high demands with regard to the ability to withstand thermal loads and temperature gradients. In the reflectors, light sources which allow a high luminous intensity to be produced within a small volume are used. The light sources are based on the technical principle of high-power halogen lamps, arc lamps or gas discharge lamps. The radiation maximum from these ultrahigh power lamps lies at wavelengths of 1 xcexcm, i.e. in the near infrared.
These glass-ceramics may be coated with metallic layers, such as aluminium, or with alternating layer systems of oxide substances. The multiple oxide layers use the interference principle and enable the visible light to be reflected while the incident infrared radiation is transmitted to the rear. The intention is for the substrate material to have a high IR transmission, so that it transmits the IR radiation to the rear without being heated to an unacceptable extent. Reflectors of this type are known as cold-light reflectors. Digital projection equipment and DVD or video recorder projection equipment are increasingly being equipped with glass-ceramic cold-light reflectors.
Glass-ceramics which are used as substrates for mirrors used in astronomy are described in DE-A-1902432 and U.S. Pat. No. 4,285,728. The shaping is produced by casting the molten glass into a refractory die. Prior to the mirror-coating, the glass-ceramics comprising beta-quartz solid solution as the predominant crystal phase which are obtained after the crystallization are initially ground and then polished. This process leads to the desired geometric contour and a low surface roughness. However, it is time-consuming and expensive.
JP-B-95037324 describes glass-ceramics made from beta-quartz or keatite solid solutions for use as reflective mirror substrate materials which, after the ceramicization, have a low surface roughness Ra of at most 0.03 xcexcm even without polishing and have a composition in % by weight which comprises 50-65 SiO2, 18 -30 Al2O3, 3-8 Li2O, 3-5 TiO2+ZrO2, 0.3-7 RO(R=Mg, Ca, Zn, Pb or V) and up to 3 R2O (R=K, Na).
U.S. Pat. No. 4,438,210 describes transparent glass-ceramics comprising beta-quartz solid solution as the predominant crystal phase, which glass-ceramics, despite having relatively high contents of Fe2O3 of up to 1000 ppm, are substantially colourless. The composition of the glass-ceramics, in % by weight, comprises 65-75 SiO2, 1-4 Li2O, 15-25 Al2O3, 0.5-2 ZnO, 0-2 Na2O and/or K2O, 2-6 TiO2, 0-2 ZrO2, 0-2.5 BaO, 0-1.2 F and 100-1000 ppm of Fe2O3.
It is an object of the invention to provide a glass and a glass-ceramic comprising beta-quartz and/or keatite solid solutions which are suitable for coating with a mirror coating, and to provide an economic and environmentally friendly process for producing the glass and glass-ceramic.
The object is achieved by a glass-ceramic having a composition in % by weight, based on the total composition, of:
and, if appropriate, at least one refining agent, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds.
The glass-ceramic according to the invention has
a low viscosity, which is advantageous for shaping by pressing, with a working point VA of  less than 1300xc2x0 C.
a good devitrification stability with an upper devitrification temperature which lies at most 50xc2x0 C. above the working point VA 
a surface roughness of the glass and glass-ceramic without polishing of Ra less than 50 nm, preferably  less than 20 nm
a thermal expansion of the glass-ceramic in the temperature range between room temperature and 300xc2x0 C. of  less than 1.2xe2x80xa210xe2x88x926/K
a high transmission on the part of the glass and the glass-ceramic in the near infrared region at 1050 nm of  greater than 85% for a thickness of 4 mm.
For shaping by pressing or blowing, the glass is to have a low working point VA of  less than 1300xc2x0 C. As a result, the thermal loads in the region of the feeder, the outlet and for the press tools are reduced, so that the service lives are increased. The low viscosity also has a beneficial effect on the melting of the glass in the melting end and on the blowing quality of the glass obtained.
To avoid undesired devitrification of the molten glass during shaping and production of the drop in the feeder, the upper devitrification point of the molten glass should lie at most 50xc2x0 C. above the working point (VA). The upper devitrification point of the molten glass is the highest temperature at which the first crystals come into contact with the shaping materials. With this temperature interval, experience has shown that it is still possible to avoid critical formation of crystals at the orifice ring or in the feeder, since the glass temperature during the conditioning of the drop is significantly above VA. It is more advantageous if the upper devitrification temperature lies below VA.
The low surface roughness of the vitreous shaped body obtained during pressing must not deteriorate to an unacceptable extent during the crystallization. Particularly when forming large mean crystallite sizes, the surface roughness of the glass-ceramic may rise. After the application of the mirror coating, the surface roughness is substantially maintained and effects partial scattering of the light. This light scattering has an adverse effect on the light efficiency. The aim for the surface roughness is for the Ra value of the glass-ceramic to be  less than 50 nm, preferably  less than 20 nm. This leads to light efficiencies which generally eliminate the need for expensive polishing of the substrate material prior to the coating.
For applications in which extremely high demands are imposed on the ability of the mirror substrate material to withstand thermal loads, the thermal expansion of the glass-ceramic in the temperature range between room temperature and 300xc2x0 C. should be less than 1.2xe2x80xa210xe2x88x926/K. This results in a high ability to withstand temperature gradients, since the temperature differences in the mirror substrate material are unable to bring about critical, thermally-induced stresses. In particular, the defects in the glass-ceramic or microcracks between mirror coating and substrate material, which cannot be avoided altogether, cannot be made to grow when the appliance is being switched on or off or in use on account of stresses caused by temperature differences.
Particularly for use as a cold-light reflector, in applications involving extremely high radiant power, the material must have a high transmission in the near infrared region, so that it transmits the IR radiation to the rear without being heated to an unacceptable extent. The composition according to the invention results in an IR transmission of  greater than 85% at 1050 nm and a thickness of 4 mm. The radiation maximum of the most intensive light sources lies at this wavelength. Unfortunately, in glasses and glass-ceramics of the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 type, there is also an absorption band at this wavelength, which is attributed to the divalent iron Fe2+. To reduce the absorption in this critical range, therefore, very clean mix raw materials, i.e. with low levels of iron, need to be selected. The preparation of the cutlet and the entire process must also keep contamination caused by iron at a low level. In both cases, this leads to increased outlay and therefore it is economically disadvantageous. Oxidic melt management by the use of nitrates as raw materials for the mix is only to a small extent able to oxidize the harmful Fe2+ to form Fe3+. The composition according to the invention leads to a good IR transmission of  greater than 85% at 1050 nm, with economically acceptable Fe2O3 contents of up to 200 ppm.
The composition according to the invention of the glass-ceramic comprising beta-quartz and/or keatite solid solutions contains, in % by weight, based on the total composition:
and, if appropriate, at least one refining agent, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds.
The oxides Li2O, Al2O3 and SiO2 are necessary constituents of glass-ceramics comprising beta-quartz and/or keatite solid-solution phases. MgO, ZnO and P2O5 may be incorporated in the crystal phases as further components. Li2O contents of over 5.5% by weight increase the rate of crystal growth and put the devitrification stability at risk. The MgO content is limited on account of the increased discoloration in combination with Fe2O3 trace concentrations. Zn, which is related to Mg in crystal chemistry terms, is also limited for this reason. The sum of the MgO and ZnO should be less than 0.3% by weight. The P2O5 content is limited to at most 8% by weight. Higher levels lead to a significantly reduced chemical stability of the glass-ceramic. This is disadvantageous since the glass-ceramic substrate materials are usually chemically cleaned before being coated with the mirror coating, in order to remove surface contamination, in particular of an organic nature. If the chemical stability of the glass-ceramic is poor, the surface of the glass-ceramic may be attacked, with the result that the surface roughness deteriorates and the light efficiency is reduced. The Al2O3 content should be 19-27% by weight.
The Al2O3 content is less than 27% by weight in order to avoid high viscosities of the molten glass and the undesired devitrification of the molten glass to form mullite. The SiO2 content is limited to at most 66% by weight, since this component increases the viscosity of the glass and impairs the relatively low working points VA of the molten glass, which are more advantageous for shaping by pressing. The addition of the alkali metal oxides Na2O,K2O and of the alkaline-earth metal oxides SrO, BaO improves the melting properties and the devitrification properties of the glass during production. To achieve the desired low working points VA less than 1300xc2x0 C., the sum of Na2O+K2O should be at least 0.5% by weight. The use of Na2O and K2O is necessary in order to produce a vitreous surface layer, which is advantageous for the low surface roughness, in the glass-ceramic. Higher levels of Na2O, K2O, SrO, BaO and B2O3 than the limits indicated cause unacceptable deterioration of the thermal expansion. This can be explained by the fact that these components remain substantially in the residual glass phase of the glass-ceramic. As residual glass-forming agents, higher levels may also impair the crystallization behaviour. TiO2 and ZrO2 are important as nucleating agents. The sum of the nucleating agents TiO2+ZrO2 should be 3.0 to 6.0% by weight. Minimum levels of 3.0% by weight are necessary in order to achieve the high nucleus density which is required for high light transmission and infrared transmission and therefore small crystallite sizes. The small crystallite sizes are also required in order to achieve a low surface roughness of the glass-ceramic without polishing of Ra less than 50 nm, preferably  less than 20 nm. The amount of nucleating agents should not exceed 6.0% by weight, since otherwise the devitrification stability of the molten glass during shaping by pressing deteriorates. The composition may contain up to 0.6% by weight of F as substitute for O. The addition of fluorine has proven appropriate in order to reduce the viscosity of the molten glass and may also increase the strength of the glass-ceramic, but even contents of 0.6% by weight lead to a deterioration in the temperature/time load-bearing capacity (compaction). It may also lead to spalling as a result of changes at the surface of the glass-ceramic.
The glass melts are refined using the refining agents which are customary for this glass system, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds in the customary quantities of 0.5 to 2% by weight.
Depending on the raw materials selected for the mix and on the process conditions during the melting, the water content of the glasses according to the invention is usually between 0.01 and 0.06 mol/l.
In a preferred embodiment, the glass is converted into a glass-ceramic comprising beta-quartz solid solutions as the main crystal phase. The glass-ceramic contains the following composition in % by weight, based on the total composition:
and, if appropriate, at least one refining agent, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds.
The glass-ceramic is distinguished by a particularly low thermal expansion of  less than 0.5xe2x80xa210xe2x88x926/K in the temperature range from 20xc2x0 C. to 300xc2x0 C. and a very high transmission in the near infrared region, for a thickness of 4 mm, at 1050 nm of  greater than 87%, preferably  greater than 89%. The Fe2O3 content should be reduced to below 130 ppm by selecting low-iron raw materials for the mix. The contents of alkaline metals, alkaline-earth meals, fluorine and the sum of the nucleating agents TiO2+ZrO2 are limited. The composition should contain in total B2O3+P2O5 1-8% by weight. These conditions lead to crystallization to form a glass-ceramic which contains beta-quartz solid solutions with small crystallite sizes and the desired properties.
To achieve a preferred object of the invention of providing a glass-ceramic which, through selection of the production conditions, contains almost exclusively beta-quartz solid solutions as crystal phase or contains almost exclusively keatite solid solutions, the crystallization temperature of the beta-quartz solid-solution phase and the temperature of transition to the keatite solid solutions should be at least 40xc2x0 C., preferably more than 80xc2x0 C. apart. This object is achieved by reduction, in particular in the case of the residual glass-forming agents, the alkali metals, the alkaline-earth metals, B2O3. The composition should also be free of added fluorine. According to this preferred embodiment, the glass-ceramic contains a composition, in % by weight, based on the total composition:
and, if appropriate, at least one refining agent, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds.
A glass-ceramic which is particularly advantageous for shaping by pressing has a low working point VA of  less than 1270xc2x0 C. and an upper devitrification temperature which is close to or even lower than the working point VA. Crystal phases which are critical with regard to devitrification are primarily mullite (aluminium silicate), baddeleyite (ZrO2). For an improved devitrification performance of this nature, it is necessary for the constituents of this critical crystal phase, in particular Al2O3, SiO2, ZrO2, to be limited, while an increase in the levels of the alkali metals Na2O, K2O, and of the alkaline-earth metals SrO, BaO has a positive effect on the devitrification behaviour. The P2O5 content should be at least 1% by weight, since P2O5 counteracts the devitrification of mullite. The composition is free of added fluorine. The preferred glass-ceramic having these properties contains a composition, in % by weight, based on the total composition, of:
and, if appropriate, at least one refining agent, such as As2O3, Sb2O3, SnO2, CeO2, sulphate and chloride compounds.
To achieve the required low surface roughness after the conversion of the glass into a glass-ceramic, without polishing, of Ra less than 50 nm, preferably  less than 20 nm, the mean crystallite size of the glass-ceramic should be  less than 300 nm, preferably  less than 80 nm. In this case, with a glass-ceramic which contains beta-quartz solid solution, it is generally possible to achieve smaller crystallite sizes, since the transition to keatite solid solutions makes the microstructure become more coarse. If the crystallites are directly at the surface of the substrate material, they are of decisive importance for the surface roughness.
The result of the composition according to the invention comprising the alkali metal oxides Na2O, K2O is that a vitreous surface layer which is enriched with these components is formed during the ceramicization, with a thickness of up to 1.5 xcexcm. Additions of the alkaline-earth metal oxides SrO, BaO and B2O3 also assist the formation of the vitreous surface layer. Greater thicknesses should be avoided, on account of the risk of surface cracks caused by differences in the thermal expansion. The vitreous surface layer has the effect of making the increase in surface roughness in the glass-ceramic compared to the starting glass less than 10 nm, generally less than 5 nm. The surface roughness of the substrate material is then determined primarily by the surface roughness produced by the process conditions employed during pressing.
In some applications, it is desirable to dye the glass-ceramic in the visible region. The advantages of high transmission in the near infrared region from 900 to 1800 nm for use as a cold-light reflector should be maintained. The good light transmission in order to achieve reliable quality assessment of the pressed vitreous shaped body should also be maintained. The coloured oxide V2O5 in amounts of from 0.1 to 0.5% by weight has proven particularly suitable for the combination of good light transmission in the vitreous state, colouring of the glass-ceramic in the visible region and high transmission of the glass-ceramic in the near infrared region. The addition of V2O5 leads to a slightly green colour in the glass, and strong colouring occurs during the ceramicization, while there is only little absorption in the near infrared.
The maximum ability of the glass-ceramic to withstand the thermal loads is determined by the compaction. Compaction means that the regions of the glass-ceramic substrate material which are exposed to high thermal loads contract to a greater extent than regions which are exposed to lower thermal loads. This effect occurs as a result of changes in the microstructure as a function of the temperature/time load on the glass-ceramic. The different temperature/time load and associated compaction makes its presence felt in particular in the case of relatively large items. A high compaction leads to unacceptably high compaction stresses between the regions of the article which are exposed to high thermal loads and those which are subject to lower thermal loads. In extreme circumstances, this may lead to fracturing after prolonged use at elevated temperatures. The compaction, based on a specimen length of 100 mm, should be less than 60 xcexcm after conditioning at 600xc2x0 C., 200 h, since this would satisfy even applications involving extremely high thermal loads.
For use as a cold-light reflector, it is advantageous if the IR transmission is  greater than 85%, preferably  greater than 87%, for a thickness of 4 mm, not only at the wavelength or 0.50 nm but also over the entire range from 900 to 1800 nm. This broad wavelength range corresponds to the radiation maxima for most light-intensive types of lamp and their spectral distribution. This makes the glass-ceramic substrate material versatile in use.
For applications in which the demands imposed on the ability to withstand temperature gradients and thermal loads are not so extreme, it is economically advantageous to leave the substrate material in the vitreous form, in order to eliminate the process step of ceramicization. To satisfy requirements, the glass should have a thermal expansion in the temperature range between room temperature and 300xc2x0 C. of  less than 5xe2x80xa210xe2x88x926/K, preferably  less than 4.5xe2x80xa210xe2x88x926/K, and a transformation temperature Tg of greater than 600xc2x0 C. The IR transmission should reach the high levels of  greater than 85%, preferably  greater than 87%, for a thickness of 4 mm, in the wavelength range from 900 nm to 1800 nm which are known from the glass-ceramic.
To allow reliable quality inspection of the pressed shaped bodies, the light transmission of the glass should be at least 85%, for a thickness of 4 mm. This high light transmission enables shaped bodies with defects which are relevant to safety or which may impair the specified product properties, such as the good light efficiency, to be sorted out. This is economically advantageous, since the following process steps up to and including the coated substrate material and final inspection cause additional costs. The light transmission of the glass-ceramic should be at least 50%, preferably more than 85%, measured at a thickness of 4 mm, in order to be able to identify flaws which occur during ceramicization and to enable the article to be sorted out prior to further process steps. For the high light transmission, it is necessary to reduce absorption and scattering effects. The absorption is substantially reduced by the low iron contents in combination with reduced TiO2, MgO and ZnO contents. To reduce the light scattering, it is necessary to reduce the crystallite sizes to significantly below the wavelength of the visible light and to keep the differences in the refractive index between crystal phase and residual glass phase at a low level.
In a preferred embodiment, the vitreous or glass-ceramic substrate material is in the form of a reflector, the inner contour of which approaches one or more parabolas. If the light source is arranged substantially as a spot source in the focal point of the parabola, the inner contour will be designed as a parabola, in order to achieve a parallel orientation of the reflected light. In the case of a light source which, for design reasons, has a linear extent, it is technically often advantageous for the inner contour to approach one or more parabolas.
To produce the cold-light reflector, the vitreous or glass-ceramic substrate material is coated with an IR-transmitting mirror coating. Metals, such as aluminium, are relatively unsuitable for this purpose, since they reflect in the near IR region. Layer sequences of various oxide layers which are optimized in terms of the number, sequence and thicknesses of the layers and the refractive indices of the layers enable these layer sequences to achieve good reflection in visible light but to be transmissive in the infrared for the thermal radiation of the light source. It is preferable to use layer sequences of oxides, such as SiO2 and TiO2. Vacuum vapour deposition, sputtering and preferably PICVD coating can be used as coating processes.
In the process according to the invention for the production of a vitreous substrate material, which can be converted into a glass-ceramic comprising beta-quartz and/or keatite solid solutions, for coating with a mirror coating, the shaping takes place via a feeder, in which a drop of defined weight is added to a pressing die. A ram with a smoothed surface is used to impress the required parabolic contour of the substrate material. The vitreous substrate materials are then removed from the pressing die and undergo thermal stress relief in a cooling furnace. The quality inspection for sorting out defective shaped bodies then takes place.
The glass according to the invention allows use both as a vitreous substrate material for coating and, after conversion into a glass-ceramic comprising beta-quartz solid solutions or keatite solid solutions as the main crystal phase, has a glass-ceramic substrate material with a high ability to withstand temperature gradients and thermal loads.
In the process for converting the pressed vitreous substrate material into the glass-ceramic comprising beta-quartz solid solution as the main crystal phase, the glass is nucleated at temperatures of 630xc2x0 C. to 750xc2x0 C. for a duration of  greater than 15 minutes, in order to produce high nucleus densities, and then the crystallization is carried out at temperatures of 700xc2x0 C. to 850xc2x0 C., for a duration of at least 30 minutes. On account of this process, the mean crystallite size of the glass-ceramic is less than 80 nm, and the thermal expansion in the temperature range between 20xc2x0 C. and 300xc2x0 C. is less than 0.5xe2x80xa210xe2x88x926/K.
To convert the pressed, vitreous substrate material into a glass-ceramic comprising keatite solid solution as the main crystal phase, the conversion takes place at temperatures of 780xc2x0 C. to 1000xc2x0 C., and the mean crystallite size is in this case less than 300 nm, and the thermal expansion in the temperature range between 20xc2x0 C. and 300xc2x0 C. is less than 1.2xe2x80xa210xe2x88x926/K. Glass-ceramic substrates materials with keatite solid solutions as the main crystal phase have a higher thermal expansion and therefore a reduced ability to withstand temperature gradients compared to glass-ceramic substrate materials comprising beta-quartz solid solutions as the main crystal phase. On account of the larger mean crystallite sizes, the light transmission is also lower, and a slightly translucent appearance, caused by scattering at the crystallites, can be observed. Glass-ceramic substrate materials comprising keatite solid solutions as the main crystal phase generally have an improved temperature/time load-bearing capacity (compaction). They can therefore be recommended for applications involving elevated use temperatures.
It is economically advantageous if vitreous substrate materials and glass-ceramics comprising beta quartz and keatite as the main crystal phase can be produced from the same composition, since they have different (property profiles and production costs.
The glass-ceramic according to the invention and/or the starting glass is preferably used as a reflector, in particular a cold-light reflector, as a substrate material for a mirror coating, as a supplementary plate in illumination engineering, in particular where, on account of a high luminous power, it is necessary to tolerate a high radiant heat combined with temperature differences. While the vitreous substrate material provides economically favourable solutions where there are reduced demands on the ability to withstand thermal loads and temperature gradients, the glass-ceramics satisfy extremely high demands with respect to these properties. These substrate materials have a very high infrared transmission and can therefore be used as cold-light reflectors.